The principal aim is elucidation of the immunogenetic basis of allograft incompatibilities. This will include several distinct, but conceptually related projects as follows: (1) Immunologic phylogeny and phylogeny of transplantation reactivity in particular. We shall continue to study primitive fishes (especially hagfish) and selected, advanced invertebrates (especially echinoderms and protochordates). (2) Weaker histocompatibility barriers in congenic strains of mice to identify gene-antigen-antibody relationships as a function of known histocompatibility genes. (3) Occurrence and mechanisms of specific immunoblocking (enhancement) in both normal and neoplastic tissue test systems employing genetically defined animals.